In continuous-forms printing systems, one or more marking engines are used to apply marking material (e.g., ink) onto a web of print media. The web is driven through the marking engines and into a dryer. As the web travels through the dryer, the dryer heats the web and dries the marking material onto the web. The web moves quickly across the printing system in order to enable fast printing speeds. For example, the web may travel at many linear feet per second through the printing system. This means that dryers must either occupy a large space within the print shop or use a great deal of heat to dry the web. For example, in many dryers, inked portions of the web transit the entire length of the dryer in a fraction of a second.
When dryers apply large amounts of heat to a web, there is a risk of a fire igniting along the web and escaping the dryer. To address this issue, dryers often use a tunnel that is bordered on all sides by solid walls. The web is heated as it travels through the tunnel, but the tunnel has narrow entrances which are so small that even if the paper is overheated, there is insufficient mass transfer of oxygen from the outside to enable the fire to leave the dryer via the tunnel.
FIG. 1 is a block diagram of a prior art dryer unit 110. Dryer unit 110 includes an array of heating lamps 114 which heat web 120 as it travels through tunnel 112. In FIG. 1, the radiant heat from radiant heating lamps 114 is indicated by element 116. Rollers 130 and 140 position web 120 as it enters and exits dryer unit 110, in order to tension web 120 during its transit through tunnel 112. Tunnel 112 includes openings 118, which are so narrow that any fires which start within dryer unit 110 do not have sufficient oxygen to escape along tunnel 112 and out of dryer unit 110. Openings 118 are known in the art as fire enclosure openings because they prevent fires from spreading outside of dryer unit 110.
Fire enclosure openings remain problematic in existing dryers. For example, even when tension is applied by rollers 130 and 140 to keep web 120 taut, web 120 may still experience upward and downward deflection along the scan direction as it travels through tunnel 112 (this is referred to as “flapping”). In addition, the web may exhibit wrinkling or puckering along the process direction due to excessive moisture from the applied marking material (this is referred to as “cockling”). This may cause further issues.
FIG. 2 is a block diagram illustrating limitations of a prior art drying unit. Here, only tunnel 112 of dryer unit 110 is illustrated. In FIG. 2, rollers 130 and 140 minimize deflection and deformation of web 120 at points 210 and 220, respectively. However, as web 120 travels through tunnel 112, it may deflect upward and/or downward by a small amount. Naturally, the amount of deflection depends on the distance to the nearest roller, physical properties of the web itself, travel speed of the web, and the amount of tension on the web. Because the narrow openings 118 are not very tall (e.g., only millimeters in height), web 120 may deflect into the ceiling of the fire enclosure openings formed by these openings 118 (e.g., at locations 230 and 240). This in turn smears marking material on web 120, which reduces print quality and is undesirable.
Thus, manufacturers continue to search for systems that improve the capabilities of dryers for printing systems.